Do not assign currentTime of video unless MetaData is available.

Fixes shaka-project#101
Required for Firefox's and IE's MediaElement implementations.

lib/player/stream_video_source.js

@@ -1110,7 +1110,17 @@ shaka.player.StreamVideoSource.prototype.startStreams_ = function(
   // Set the video's current time before starting the streams so that the
   // streams begin buffering at the stream start time.
   shaka.log.info('Starting each stream from', streamStartTime);
-  this.video.currentTime = streamStartTime;
+
+  function setInitialCurrentTime() {
+    this.currentTime = streamStartTime;
+    this.removeEventListener('loadedmetadata', setInitialCurrentTime);
+  }
+
+  if (this.video.readyState >= HTMLMediaElement.HAVE_METADATA) {
+    this.currentTime = streamStartTime;
+  } else {
+    this.video.addEventListener('loadedmetadata', setInitialCurrentTime);
+  }
 
   // Start the streams.
   for (var type in this.streamsByType_) {